High-speed and high-density DRAM (Dynamic Random Access Memory) has found wide application as RAM (Random Access Memory) in information equipment such as computer. However, DRAM involves high manufacturing cost for its complicated manufacturing process as compared to typical logic circuit LSI (Large Scale Integration) and signal processing used for electronic equipment. Further, DRAM is a volatile memory in which information is lost when power is removed. Therefore, it requires frequent refresh, which means that the written information (data) must be read, amplified again and rewritten.
For this reason, flash memory, FeRAM (Ferroelectric Random Access Memory) and MRAM (Magnetoresistive Random Access Memory) have been proposed, for example, as non-volatile memories which can retain information even when power is removed. These types of memories can retain the written data for extended periods even without any supply of power.
However, the aforementioned types of memories have their advantages and disadvantages. Flash memory is high in integration degree but disadvantageous in terms of operating speed. FeRAM has its limit in patterning required for high integration. This memory also has problems with its manufacturing process. MRAM has a power consumption problem.
Therefore, a new type of memory element is proposed which is particularly advantageous in overcoming the patterning limit of the memory element. This memory element has an ion conductor containing given metals sandwiched between two electrodes. In this memory element, one of the two electrodes contains the same metals as contained in the ion conductor. As a result, when a voltage is applied between the two electrodes, the metals diffuse into the ion conductor as ions. This changes the electrical characteristic, such as resistance value or capacitance, of the ion conductor. For example, JP-T-2002-536840 and Issue of Nikkei Electronics No. 20 (page 104), January, 2003, describe memory devices using this property. JP-T-2002-536840 in particular proposes an ion conductor which includes a solid solution made of chalcogenide and metal. More specifically, the ion conductor includes a material in which Ag, Cu and Zn are dissolved in AsS, GeS or GeSe. One of the two electrodes contains Ag, Cu and Zn.
However, if the ion conductor of the memory element configured as described above is left standing for extended periods in a low-resistance value stored state (e.g., “1”) or in a high-resistance value erased state (e.g., “0”), or alternatively if the ion conductor is left standing at an atmospheric temperature higher than room temperature, the resistance value thereof will change, causing the memory element to lose the stored information. Thus, a memory element is unfit for use as a non-volatile memory if it has only low data retention capability.
Further, if, in recording a large amount of information per same area, it is possible to provide an intermediate arbitrary resistance value between, for example, a high resistance state of several hundreds of MΩ and a low resistance state of several kΩ, rather than simply high resistance state of “0” and low resistance state of “1”, then the memory will have a wider operating margin and can record multivalued information. That is, if one memory element can store four resistance states, two bits of information can be stored in each element. If one memory element can store 8 resistance values, three bits of information can be stored in each element. As a result, the memory capacity can be improved two-, three- or more fold.
With known memory elements, however, if the range of variable resistance value is, for example, from several kΩ to several hundreds of MΩ, the resistance values which can be held in low and high resistance states are about 10 kΩ or less and 1 MΩ or more, respectively. Therefore, it is difficult to retain an intermediate resistance value between high and low resistance states, thus making it difficult to store multivalued information.